Why do Grandparents not bother with Grandson

I think there might be a lot more to this than can be written about here. However, your parents in law can't have it both ways. If you know it upsets them to see photos as described perhaps be a tad more circumspect but make sure the next time you see them take a pic or two to post as you see fit?
Not all grandparents are "into" being granny & grandad. Perhaps they're not that interested in small children & will bond more with your son as he grows and interacts with them?
In your shoes I'd probably ignore their issues about photos but try to acknowledge them in other ways; remember their birthdays, send photos of your son in the post with a card, keep the lines of communication open. Don't let their "issues" eat you up. Accept it for now but continue with an open heart and kindness. Turn the other cheek & continue to be the lovely DiL you clearly are. Easter is coming ...send a Happy Easter card or drop it round taking your son with you, with an Easter Egg. Soften their hearts with kindness. Who can argue with that?

Did your husband ask why they didn't like to see the photo of your parents with your son? If you have asked them more than once and they've refused, then I suggest you disengage from any contact and leave it to your husband. Tell him you don't want to hear complaints about your parents.

In the end, if you have made the effort and they haven't responded, it's on them. You should be able to post the photo. If they want a photo, they can come visit.

I know of one gran who feels left out even though she has had invitations. The type of invitations don't match with what she thinks a gran is. She thinks a gran is having the grandkids with her all weekend, without the parents, like she did with her own children. The parents want weekends with their own children and want visits to be more whole of family together. Both sides feel the other is not interested in the other because they want different things. If you've got this kind of dynamic going on, a conversation might help.

This needs a frank conversation between your husband and his parents, take yourself out of the loop. Basically, they can’t expect to have it both ways.

The photo was of my parents with my son. A comment was made to my partner when he saw them last year on his own to drop of a present saying they don’t like seeing my son with my parents. Why should I not put a photo up of them?

As a grandparent I did not interfere, and did not expect to be asked to share much of the grandchildrens' childhoods.

I’ve invited them to go to the park with their Grandson..they refused. Even my husband has said to text us or call us anytime.

They never bother to contact us even though last year I contacted them constantly. It feels like it’s a one way relationship all the time. And I’m getting fed up with it.

With regards to the photo they don’t like my parents seeing their grandson! If he didn’t see my parents he’d have no one.

You say they haven't been in touch but then say they are complaining about photos. Which is it? Perhaps they are waiting for a specific invitation? If they bother you so much I suggest you avoid putting photos on social media or block them from seeing them. It is your partner's place to contact his parents with updates etc, particularly if you are getting grief from them. Have you spoken to them about the apparent lack of interest? That might be a good place to begin.

Are they waiting for an invitation to visit, or waiting for their son to ask? Could it be they are taking not wanting to interfere or be pushy to the extreme and letting you initiate visits and communication? Can you husband have a conversation with them? A comment on a photo is a perfect opener to find out what they mean by it. Kindly of course.